1. Field of the Invention
The present invention relates to electron beam lithography and, in particular, an electron beam lithography system having an improved electron gun.
2. Description of the Related Art
The greatest limitation on electron beam lithography systems has been their speed of operation, or throughput. Multiple electron beam technology holds the potential for greatly improving throughput in the next generation (sub 100 nm) of lithography. In this approach, multiple electron beams are formed by focusing an array of independently modulated light (laser) beams onto a photocathode in transmission mode. Electron beams are emitted if the energy of the photon beam is greater than the work function of the photocathode material. Once created, the electron beams are accelerated, focused and scanned across the wafer or mask using an electron-optical column. In such systems, the selection of the photocathode material is an important performance-limiting factor.
For an electron gun employing a photocathode source, the total current that can be delivered to the wafer of substrate is limited by the quantum efficiency (QE) of the photocathode and the transmission of the electron column. The QE is the ratio of the number of emitted electrons to the number of incident photons and is largely an intrinsic property of the material. The column transmission is related to the energy distribution of the emitted electrons, such that a wider energy distribution results in a lower column transmission.
Many photocathode materials have been used. These include metallic films such as gold; semiconductor materials such as gallium arsenide; and semiconductor materials whose surfaces have been treated with cesium and oxygen, referred to as negative electron affinity (NEA) photocathodes.
Generally, the highest quantum efficiencies are produced with NEA photocathodes (as large as 30% with an energy of 100 meV). However, the QE of NEA photocathodes is very sensitive to vacuum contaminants (e.g., a vacuum of 10−11 Torr is generally required to maintain a high QE). Further, the QE of NEA photocathodes degrades with time as current is emitted from the surface. This degradation is caused by a loss of cesium from the surface due to desorption.
Gold photocathodes, on the other hand, are much less sensitive to vacuum conditions and can operate under high vacuum conditions. However, gold photocathodes typically exhibit low QE, which ultimately would result in low throughput.
There is therefore a need for an electron gun having a photocathode with relatively high quantum efficiency and that can operate at relatively high vacuum. There is a further need for an electron beam lithography system having such an electron gun.